The present invention relates generally to electronic devices, and more particularly to digital-to-analog conversion.
Modern transmitters for applications such as cellular, personal, and satellite communications employ digital modulation techniques such as quadrature phase-shift keying (QPSK) in combination with code division multiple access (CDMA) communication. Shaping of the data pulses mitigates out-of-band emissions from occurring into adjacent spectral channels but produces time-varying envelopes. Transmission of multiple carriers simultaneously also results in time-varying envelopes. In many cases the time varying envelopes exhibit large but infrequent peaks. These peaks require digital-to-analog converters (DACs) within the transmitter to have dynamic ranges that encompass the peak values to avoid clipping the signal which would introduce significant distortion of the signal. This signal distortion can lead to both errors in the wanted signals and increased out-of-band emissions.
Dynamic range refers to the ratio between the largest signal and the noise level of the DAC. Noise sources include electronic noise, quantization noise, and spurious signals resulting from non-linearities in the DAC. Dynamic range can be measured in decibels or effective bits.
A DAC provides outputs signals over a specific voltage range. To operate over a wide range either the voltage step size between outputs, and the corresponding quantization noise, is increased or dynamic range must be increased. For conventional multi-bit DACs the quantization noise is relatively constant as a function of frequency and extends from DC to one half the conversion rate. Delta-sigma DACs have very low quantization noise in selected passbands and high noise levels elsewhere. Quantization noise that is not filtered in the signal chain is amplified and transmitted contributing to received bit-errors and causing interference in adjacent spectral regions.
Recently, a new CDMA standard has evolved referred as WCDMA (wide code division multiple access) that employs multiple frequency carriers. WCDMA has signal amplitudes that vary substantially over time producing large peak-to-average ratios. Other standards that include multi-carriers or similar noise shaped signatures (e.g., Orthogonal frequency-division multiplexing (OFDM), Global System for Mobile Communication (GSM)) have similar large peak-to-average ratios. These standards are governed by increasingly strict international standards and national regulations mandating low out-of-band emissions. Therefore, DACs capable of converting signals with high peak-to-average rations and/or with large dynamic range are required in applications making use of these standards in light of their large associated peak to average signal ratio.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to a system for converting a digital input signal into an analog output signal. The system is operative to alter the instantaneous dynamic range of a system of one or more DAC devices in response to a characteristic of the input signal to mitigate the quantization noise produced in the conversion.
In one aspect of the invention, a conversion system converts an input signal from a digital signal to an analog signal over an extended dynamic range. The system includes a DAC system and a mode selector. The mode selector selects a mode of operation for the digital-to-analog converter system from a plurality of modes. The mode selector selects a mode according to a characteristic of the input signal. Each of the modes is associated with an instantaneous dynamic range and a quantization noise level of the DAC system. In a further aspect of the invention, the DAC system can include one or more delta-sigma modulators to convert the digital input signal to a digital signal having a higher frequency prior to analog conversion.
In another aspect of the invention, the system for converting a digital input signal into an analog signal can include a DAC system comprising a plurality of DAC devices. Each of the DAC devices has an associated dynamic range and quantization noise level. The mode selector includes a switch that selects the output of one of the plurality of DAC devices based upon a characteristic of the input signal. In a further aspect of the invention, the signal characteristic can be the amplitude of the input signal.
In yet another aspect of the present invention, the system for converting a digital input into an analog signal can include a DAC system comprising a DAC device with a variable dynamic range. The DAC device includes a voltage supply input that allows the input voltage range of the DAC device to be altered. The mode selector alters the dynamic range of the DAC device according to a characteristic of the input signal.
In still another aspect of the invention, a system for converting a digital input into an analog signal can include a DAC system comprising a DAC device, a scaling device, and an extra gain switch. A mode control determines an appropriate scaling factor from a set of at least one discrete scaling factor according to a characteristic of the signal. The selected scaling factor can be provided to the scaling device and according to a control signal from the mode control. A corresponding gain factor can be provided to the extra gain switch. The input signal can be scaled by the scaling device before it is input into the DAC device. The output of the DAC device can be amplified by the selected gain factor.
In a still further aspect of the invention, a system for converting a digital signal to an analog signal can include a mode control/select and a DAC system. The mode control/select splits the input signal into a phase modulated component and an amplitude modulated component when a characteristic of the input signal meets a predetermined threshold. A first DAC device processes the amplitude modulated component and a second DAC device processes the phase modulated component. The output of the second DAC device is amplified by an amplifier receiving the output of the first DAC device as a supply voltage input. Where the threshold is not met, the entire input signal is provided to the second DAC device and a constant voltage supply signal is applied to the amplifier.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.